A printed circuit board (PCB) is a board of laminated insulator material with routing channels, also called conductive traces, on the surface and embedded in the PCB. The routing channels interconnect the components (for example, transistors, diodes, resistors, LEDs, connectors, etc) that are placed on the surface(s) of the PCB. The main body of the PCB is made up of different layers laminated together. The laminate material is an electrical insulator and can be made of e.g. epoxy and fiberglass. The electrical connections on the PCB are practically always made up of copper, e.g. the routing channels are made up by strips of copper, called micro strips on the outer layers and strip lines on the inner layers, connecting different components. A layer of the PCB that contains routing channels is called a conductive layer or a signal layer. A PCB can have a plurality of conductive layers, wherein the routing channels of the inner conductive layers are buried inside the insulator. In between different conductive layers, a ground layer, which is a layer that is entirely at ground potential, may be arranged.
In order to connect different conductive layers of a PCB so called via holes are arranged in the PCB. Via holes are conductively plated on their interior surface and routing channels on the conductive layers may be connected to the via holes through a so called pad surrounding the via hole. Via holes can protrude all the way through a PCB or they can be buried, which means that the via holes interconnects internal layers and cannot be seen from the exterior of the PCB. Via holes can also be blind, which means that they can be seen from one side of the PCB. When fabricating multilayer PCBs having a plurality of conductive layers, often the pads in different layers are connected using a special copper plating process commonly referred to as a plated through hole (PTH). PTHs allow interconnectivity between layers on via holes and are produced/drilled after the different layers of the PCB are pressed together. In ground layers the PTHs that are carrying signals are surrounded by so called anti-pads, which basically is copper that has been etched away around a via or a PTH on a ground layer and thereby prevents electrical connection to the layer from the via or PTH.
PCB's usually have a foot-print of PTHs for press-fit or soldered connectors that have a predefined hole pattern. Pads that surround PTHs have a specified diameter to handle production tolerances and also ensure connection to the inner layers in the PCB. The pads have a minimum diameter depending on the drilled diameter of the PTH, which is specified depending on the connector type. Circular pads are normally used for plated through holes in PCBs. Each connector type has a unique connector foot-print consisting of PTHs in the PCB for the signal pins and the ground pins.
In the (near) future products will be based on high speed technologies for bit rates above 10 Gb/s single lane. For a successful transmission link all interconnection elements require optimized electrical performance. A transmission link may comprise transceivers, connectors and PCBs. The layout of current PCBs is adding discontinuities which are reducing the signal quality. PTHs, impedance mismatch of transmission lines, bends on routing channels, ground return coverage in adjacent layers, crosstalk, etc., are some factors that reduce the signal quality.
For high speed connectors the number of signal interfaces per area is increasing and therefore also the density of the connector foot-print to an associated PCB is increasing. The routing channels should then be made as large as possible since small routing channels through the connector foot-prints results in reduced signal performance depending on crosstalk, impedance mismatch, increased insertion loss, etc. Large routing channels result in that the anti-pads should be made as small as possible. Large anti-pads make manufacturing easy but reduce the area in which routing channels may be arranged in adjacent layers, since it is not preferred to route high speed channels over a ground layer having large anti-pads where the routing channels may be passing over an area in an adjacent layer without copper. However, small anti-pads lead to discontinuity at PTHs depending on the capacitance between the pad and an adjacent ground layer.